328 SCIENCE PROGRESS. 



pansion. Schizonotus, which is a most remarkable but 

 little known form, superficially resembling Thelyphonus but 

 with a jointed cephalothorax and the large helmet-plate 

 like Galeodes, appears to have the first four segments fused. 

 Scorpio has the first six segments fused, and no external 

 trace of any waist, although, internally, the constriction 

 between the sixth and seventh segments has been drawn 

 in to form a diaphragm ; all the segments behind the 

 seventh are free. Seven of these are swelled up to form a 

 distensible bag, while the last five show an exactly opposite 

 specialisation ; they are thin, drawn out, thickly armoured 

 and firmly hinged to one another to form an almost invul- 

 nerable tail, the terminal joint of which is armed with a 

 sting. Thelyphonus has the first six segments fused rigidly 

 together so as to form a long narrow cephalothorax, a waist 

 between segments six and seven, nine abdominal segments 

 free and forming a distensible bag, and three minute 

 telescoped segments forming a sort of short tail which carries 

 a long whip-like appendage. Phrynus differs from Thely- 

 phonus in having the first six fused segments so compressed 

 longitudinally that the cephalothorax is as broad or broader 

 than it is long, while the tail segments are more telescoped 

 and not furnished with any caudal appendage. The Spiders 

 have the first six segments fused into a compact round or 

 oval cephalothorax, a waist between the sixth and seventh 

 segments, and all the abdominal segments fused together. 

 These latter nevertheless form a distensible bag, but not 

 by any means so distensible as in the case of the other 

 Arachnids with freely telescoping abdominal segments. 

 The Book-scorpions have the first five or six segments 

 fused, no waist visible, all the abdominal segments free and 

 forming a distensible bag. The other families, Harvest- 

 men and Mites, departing somewhat far from the leading 

 types here described, need not detain us. 



Now, of these various fusions and specialisations of seg- 

 ments Galeodes, with only three fused segments, shows far 

 and away the least modification of an assumed freely seg- 

 mented ancestor. No other Arachnid (except Schizonotus 

 and, perhaps, some of the Book-scorpions) has less than six, 



